Hi,
Thank you very much for the details, I hope I understood everything correctly.
1. The .htaccess problem
That happens, when there is either an incorrectly set mod_rewrite redirection rule in the .htaccess file, or there is different rewrite rule in effect – it can be set by a plugin, or maybe the general permalink structure is incorrectly set.
The issue is triggered, whenever the ajax handler cannot be accessed. This issue is unfortunately very hard to debug.
– First, I suggest checking the https://yoursite.com/wp-admin/admin-ajax.php URL. Copy and paste that to the browser, obviously change the yoursite.com to your domain, and see what comes up. The results should be a white page with either 0 or -1 printed to it. If it’s not, then there is a rewrite rule defined somewhere.
– Then check if the site URL is set correctly across the WordPress back-end, usually www and non-www versions of the site are the problem – it should be the same everywhere.
2. Well, this is only possible via custom coding. Dynamically updating/conditional filters are not yet implemented. Probably using a single price filter, and then changing the price filter key based on the selection (via a custom code) is the easiest solution.